The present invention is directed to a direct current arc furnace. More particularly, it relates to a direct current arc furnace which has a cathodically connected graphite electrode adjustable relative to a hearth and a container with a base forming an anode.
In direct current arc furnaces, in contrast to three-phase arc furnaces, it is possible to work with only one graphite electrode as cathode and one ground electrode as anode, which graphite electrode can be adjusted through the cover of the furnace. This results in lower construction costs with respect to the plant. Such direct current arc furnaces are also distinguished by substantially lower electrode consumption as well as by a longer service life of the refractory lining of the container wall. With respect to energy the consumption is lower; moreover network reactions (flickering) are noticeably reduced. Due to decreased inductive influences the use of austenitic steel qualities can also be dispensed with to a great extent in the construction of the furnace. Another advantage of the direct current arc furnace consists in its lower noise emissions.
Certain problems are posed in the direct current arc furnace by the anodically connected bottom electrode forming the backplate electrode for the cathodically connected graphite electrode which can be adjusted relative to the hearth through the cover of the container. An iron core penetrating through the base and through the refractory lining arranged on the base was first provided for this purpose. However, the iron core is subjected to a rapid melting away especially in melts with low carbon and high oxygen contents. Further, instead of an iron core with a comparatively large cross section, a plurality of steel rods were provided extending from an anodically connected steel plate arranged at the base of the container up to the hearth base (DE-OS 34 09 255). In this respect the lining, i.e. the insertion or application of the refractory lining, poses familiar difficulties. Difficulties are also posed by subsequently driving the steel rods into the refractory lining until contacting the anodically connected base plate. Insofar as it has been suggested to construct the area of the container base forming the anode so as to be exchangeable (DE-OS 35 35 692) corresponding problems arose in the construction of the exchange element. Moreover, in both cases the construction of the anode area remained limited to the center of the hearth base.
Instead of the steel rods forming the anode it was also suggested to form the refractory base lining itself so as to be electrically conducting by inserting stones encased in sheet metal or graphite or stones with a higher proportion of graphite. A copper insert in the form of rails or plates was provided as intermediate layer between the steel container base and the refractory lining, the anode current being applied to this copper insert (DE-OS 35 34 750 and DE-OS 34 13 745). A variant provides for a multiple-layer refractory lining of the container base having a plurality of layers of refractory electrically conducting stones and a refractory stamping mass arranged on the latter. Steel rods contacting the uppermost stone layer are driven through the stamping mass (DE-OS 29 05 553). The refractory base lining can also be provided with an anodically connected copper plate as underpinning. However, a copper plate itself is not suitable for taking over supporting functions. For this reason a sheet steel base is provided according to the prior art which is provided with a cover of abutting copper sheet segments which are connected with the sheet steel base by a plurality of pins.
The construction of the container base itself as anode proves advantageous to the extent that the current conduction is effected with the greatest possible contact surface resulting in a low specific current loading of the container base, specifically its refractory lining, which is reflected in an improvement of the service life of the base. The manner in which the supporting sheet steel base is connected with the copper covering proves disadvantageous in this prior art. For this purpose a plurality of bore holes are to be introduced into the sheet steel base and into the copper sheet segments assigned to it with a matching hole pattern. The pins connecting the elements extend through these copper sheet segments. This manner of connecting the sheet steel base and copper sheet segments is obviously very costly. Air gaps also necessarily remain between the sheet steel base and the copper covering and between the copper sheet segments themselves as well as between the supporting sheet steel base and the copper segments which also undergo an enlargement in the course of operation under the influence of the high operating temperature. This results in the risk that heavy metal proceeding from the insert will run under the copper covering. The required cooling of the base is accordingly impaired and ultimately also the life of the base.